The present invention relates generally to bone fixation devices, and more particularly, to improved bone screw constructs.
The prevalence of degenerative bone conditions requiring surgery and screw and rod support structures has steadily increased over the past four decades. Degenerative spinal conditions are becoming particularly prevalent in recent years, with pedicle screw and rod systems having become the most common treatment to obtain thoracolumbar spinal fusion worldwide. The incidence of spinal surgery and other bone fixation procedures in the United States is expected to further increase even more rapidly as more and more members of the “baby boomer” generation reach age 65. However, the rapidly growing aging population presents a challenge to effective and durable screw fixation because the increased rigidity of older bones increases pull-out stress on bone screw constructs, while the poor bone quality of osteopenic and osteoporotic patients decreases the ability of screw structures to maintain firm purchase with such bones. As a result, screw loosening and bone fusion failure occur at nearly ten times the normal rate in elderly and osteoporotic patients, representing a significant health detriment to the aging bone fusion population at large and a financial burden to the U.S. healthcare system.
Currently, the most commonly practiced solutions to poor bone screw purchase in osteoporotic and rigid aging patients are: 1) bone cement augmentation of the bone either prior to bone screw insertion or through a cannulated bone screw once placed; 2) increased bone screw thread count and unique thread designs to better purchase the screw within the desired bone walls; 3) expansion of the distal end of the bone screw to disallow unidirectional complete screw pull-out; 4) use of less rigid or dynamic rods to decrease pull-out forces on the screw-bone interface; and 5) more aggressive external bracing of the patient's body in the post-operative period. Despite these advancements, bone screw fixation failures and pseudo arthrosis remain a common complication in the treatment of elderly and osteoporotic patients.
In particular, current pedicle screw systems aim to use outer screw threads to partially cut into the inner cortical bone rim of the pedicle while the center of the pedicle screw sits in the very soft cancellous bone center. This concept relies on the surgeon to place the ideal pedicle screw diameter to best fit the pedicle diameter to maximize the fixation force between the screw thread-inner cortical bone interface. This remains a sub-optimal solution because the inner cortical bone wall-screw thread interface often provides sub-optimal screw fixation that may allow bidirectional screw movement within the bone when stressed several thousand times per month in the average mobile patient. Even when optimal screw thread-inner cortical bone rim purchase is achieved at surgery, the osteoporotic inner cortical rim is often not rigid or durable enough to maintain the desired thread-bone fixation.
What is needed, then, are improved bone screw systems that help provide increased multi-directional pull-out strength and durability so as to better purchase and fixate various bones within a patient.